This circuit is
a Schmitt
Trigger, a type of comparator, built using a
555 timer
chip
for some reason. It measures the input to see
if it is above or below a certain threshold. The threshold varies to
make it less likely that the output will switch rapidly back and forth
due to a noisy input near the threshold.

This circuit uses the fact that a 555 output goes high when the
trigger input goes below 1/3 Vin, and the output goes low
when the threshold input goes above 2/3 Vin.